Method for implementing power equalization of dense wavelength division multiplexing system

ABSTRACT

The invention discloses a method for implementing power equalization of a DWDM system, comprises: Measure and calculate, respectively, a gain spectrum characteristic curve of an optical power booster amplifier unit and a loss spectrum characteristic curve of a loss device with related wavelengths in the DWDM system; Subtracting the loss spectrum characteristic curve from the gain spectrum characteristic curve to obtain a difference curve, taking the complement curve of the difference curve as a loss characteristic target curve of a GFF; Setting in the optical power booster amplifier unit a GFF having loss characteristic curve coinciding with the loss characteristic target curve. The invention synthetically considers gain spectrum of an optical power booster amplifier unit and loss spectrum of a loss device with relating wavelengths to define a loss characteristic curve which a GFF should have. In this way, optical power flatness of every channel is effectively guaranteed. Therefore, transmission link flatness of whole system is guaranteed, and channel optical power equalization property of a system is further improved.

This application is a 371 of PCT/CN02/00442 filed Jun. 26, 2002.

FIELD OF THE TECHNOLOGY

The present invention relates generally to an optical power equalizationmethod for every channel in a Dense Wavelength Division Multiplexing(DWDM) system, and more particularly to an implementing method forimproving transmission gain spectrum flatness of a DWDM system.

BACKGROUND OF THE INVENTION

Along with the rapid development of digital communication, modemtelecommunication system has a relentless demand for networks of highercapacities. In optical communication area, capacities of optical fibersare tremendous. In traditional transmission networks, not matter it isspace-division multiplex (SDM) or time-division multiplex (TDM) forexpanding capacity, it is only a single wavelength transmission ofoptical signals. The bandwidth of optical fiber comparing with thesingle wavelength transmission is almost unlimited. In order to deploythe tremendous bandwidth resource of optical fiber and to increasecapacity of optical fibers transmission, a new generation optical fibertechnology, taking Dense Wavelength Division Multiplexing (DWDM)technology as a core, has been naturally developed.

Deploying the widthband and low loss properties of a single mode opticalfiber, DWDM technology uses multiple wavelength optics as carriers, andallows optical signals with different wavelengths propagatessimultaneously in an optical fiber. Conventionally, optical channelskept farther apart (larger spacing) and even multiplex at differentwindows of a fiber is called Wavelength Division Multiplexing (WDM), butchannels kept less apart (smaller spacing) and multiplex at same windowis called Dense Wavelength Division Multiplexing (DWDM). At present,wavelength spacing of multiplex can be nanometer level or even severaltenth of one nanometer. Comparing with single channel system, DWDMtechnology not only greatly increases network capacity and thoroughlyuses fiber bandwidth, but also has advantages such as simplicity ofexpanding capacity and reliability etc. Especially, the DWDM is capableof accessing multiple services directly, so it will have a brightapplication future.

Appearance of Erbium-Doped Fiber Amplifier (EDFA) makes that thewavelength division multiplexing technology develops rapidly. With thistechnology, increasing communication traffic needs only increasing moremultiplexing wavelengths. Nevertheless, more multiplexing wavelengthsneed that amplifier has wide and flat gain spectrum, but gain spectrumof EDFA is not so flat as expected. At present, increasing multiplexingwavelengths is mainly restricted by gain bandwidth of EDFA used in asystem. In 1545˜1560 nm wavelength band, gain spectrum of EDFA areflatter, so in general, there is no need to flatten with Gain FlattenedFilter (GFF). However, for wider wavelength band (such as 1530˜1560 nmwavelength band), because of gain spectrum characteristic of EDFA, it isneeded to flattened the gain with GFF. As a filter, it is required thatGFF inserting loss is different for different wavelengths. If lossspectrum curve of GFF coincides with gain spectrum curve of EDFA, thenwider and flatter gain spectrum can be obtained. Of course, thisflattening is cost by power loss.

FIG. 1 shows a present DWDM system, which mainly includes an opticalmultiplexer 101, an optical power booster amplifier module 102, atransmission fiber 103, an optical link amplifier unit 104, atransmission fiber 105, an optical preamplifier unit 106 and an opticaldemultiplexer 107. Among them, the optical preamplifier unit 106 and theoptical link amplifier unit 104 are basically the same, which mainlyinclude an optical preamplifier (PA) module 108, adispersion-compensating module (DCM) 109 and an optical power boosteramplifier (BA) unit 110. The multichannel signals are combined atoptical multiplexer 101, then pass optical power booster amplifiermodule 102, and enter transmission fiber 103. After some distances alongthe fiber, the signal enters optical link amplifier unit 104, because ofcompensation for power fading and dispersion. In general, the opticallink amplifier unit is consisted of PA 108, DCM 109 and BA 110, amongthem DCM 109 mainly is a dispersion compensating fiber and is optional.After signal power has been amplified and dispersion has beencompensated, the signal enters transmission fiber 105 again. Afterseveral stages of similar link, the signal enters optical preamplifierunit 106 for power amplify and dispersion compensation.

The disadvantages of present technology are: the design of GFF of EDFAused in DWDM system, only considers absorption spectrum of EDFA itselfis non-flatness, without considering loss spectrum of transmission fiberor dispersion compensating fiber used in higher than 10 Gb/s speedsystem. If the loss spectrums of these fibers have more different underdifferent wavelengths, the difference will be accumulated along withincreasing of length, and will affect power equalization of a system.The main reason is that every channel optical power difference affectedby optical fiber will increase along with length increasing. Withoutconsidering this phenomenon, optical power equalization between everychannel is getting worse, when passing links are increased. Taking Leaffiber of Corning Co. as an example for measuring, the results are asfollow. FIG. 2 shows loss spectrums measured for 25 km single modefiber. FIG. 3 shows loss spectrums measured for dispersion compensatingfiber of 60 km single mode fiber. It can be seen from FIG. 2 thatmaximum power difference, inserted by 25 km single mode fiber, betweenchannels is greater than 0.3 dB. Suppose a system has 8*22 dB distanceswith a 640 km single mode fiber; for 32 channels, the maximum differenceof loss inserted by the 640 km single mode fiber will be 7.68 dB. FIG. 3shows that, for 32 channels, insertion loss difference of dispersioncompensating fiber, used to compensate 60 km single mode fiber, isapproximately 1 dB. Similarly, for 32 channels in 8*22 dB system,insertion loss difference of dispersion compensating fiber, used tocompensate 640 km single mode fiber, will reach 10 dB.

SUMMARY OF THE INVENTION

The invention proposes a method to raise effectively optical powerflatness for a DWDM system, in order to guarantee optical powerequalization of every channel.

A method for implementing power equalization for a DWDM system comprisesthe steps of:

a) Measure and calculate, respectively, a gain spectrum characteristiccurve of an optical power booster amplifier unit and a loss spectrumcharacteristic curve of a loss device, having related wavelength withthe optical power booster amplifier unit, in a DWDM system.

b) Subtract the loss spectrum characteristic curve from the gainspectrum characteristic curve to obtain a difference curve. Then,complement the difference curve to obtain a complementary curve. Thecomplementary curve is defined as a loss characteristic target curve ofa GFF.

c) Set a GFF having loss characteristic curve coinciding with the losscharacteristic target curve in the optical power booster amplifier unit.

The method further comprises that divide, in advance, the whole DWDMsystem into more than one independent unit which includes at least theoptical power booster amplifier unit. Then, calculate the losscharacteristic target curves for GFF of every independent unit itself,respectively.

The method also further comprises: having defined an EDFA as the opticalpower booster amplifier unit; having defined a transmission fiber, or adispersion compensating module, or their combination as the loss devicewith relating wavelength; having defined the dispersion compensatingmodule is consisted of dispersion compensating fibers.

The invention considers gain spectrum characteristics and loss spectrumcharacteristics synthetically. The invention not only considers gainspectrum non-flatness of optical power booster amplifier units, butconsiders loss spectrum non-flatness of transmission fibers anddispersion compensating modules as well. The invention makes losscharacteristic curve of a GFF is a complement of a combined gain lossspectrum characteristic curve. The invention divides a whole DWDM systeminto several independent units, and defines loss characteristics curveof every unit GFF, respectively. In this way, optical power flatness ofevery channel in each unit is effectively guaranteed, so transmissionlink flatness of whole system is also guaranteed, and channel opticalpower equalization property of whole system is greatly improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general DWDM system block diagram.

FIG. 2 is a measured loss spectrum diagram for 25 km G.652 single modefiber.

FIG. 3 is a measured loss spectrum (taking Leaf fiber as an example)diagram of a dispersion compensating fiber used to compensate dispersionof 60 km G.652 fiber.

FIG. 4 is output optical power diagram of PA and BA.

FIG. 5 is a block diagram of an embodiment DWDM system of the invention.

FIG. 6 is an insertion loss spectrum diagram of a 130 km G.652 fiber.

FIG. 7 is an insertion loss spectrum diagram of a dispersioncompensating fiber used to compensate dispersion of an 80 km single modefiber.

FIG. 8 is an insertion loss spectrum diagram of a GFF designed under twoconditions: with fiber affection and without fiber affection.

FIG. 9 is a comparison diagram of power equalization characteristics forevery channel of a system when the GFF is designed under two conditions:with fiber affection and without fiber affection.

FIG. 10 is a block diagram of another embodiment DWDM system of theinvention.

EMBODIMENTS OF THE INVENTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout.

FIG. 5 shows an embodiment of the invention, which is a DWDM systemwithout link amplifier unit and transmission by a 130 km G.625 fiber.The system mainly includes an optical multiplexer 101, a dispersioncompensating module 501 used to compensate dispersion of a 20 km singlemode fiber, an optical power booster amplifier module (BA) 502, atransmission fiber 503, an optical preamplifier unit 504 and an opticaldemultiplexer 107. Among them, the optical preamplifier unit 504 mainlyincludes: an optical preamplifier module (PA) 505, adispersion-compensating module (DCM) 506 used to compensate dispersionof a 80 km single mode fiber and an optical power booster amplifiermodule (BA) 507. As there is no link amplifier unit, the system can bedivided into two parts: transmitting end unit 508 and receiving end unit509, to define loss characteristics of each part GFF.

For 10 Gb/s or higher then 10 Gb/s system, in order to have somedispersion allowance, a dispersion compensating fiber to compensatedispersion of 100 km single mode fiber is inserted to the system. Inthis embodiment, the dispersion compensating module 501 for compensatingdispersion of 20 km single mode fiber is put on transmitting end unit508, and the dispersion compensating module 506 for compensatingdispersion of 80 km single mode fiber is put on receiving end unit 509.

At transmitting end unit 508, after combining multichannel signals inoptical multiplexer, the signal passes through the dispersioncompensating module 501 for compensating 20 km single mode fiberdispersion, and enters the optical power booster amplifier module 502.In this embodiment, as the length of dispersion compensating fiber forcompensating 20 km single mode fiber dispersion is shorter, so it lossspectrum can be neglected. The GFF in BA 502 can only consider itselfflatness, and affection of successive transmission fiber will beconsidered later. Therefore, the GFF loss characteristic in thetransmission fiber 503 and receiving end unit 509 can be defined bysynthetically considering the following factors: final flatnesssituation of former BA 502, 130 km transmission fiber 503, non-flattenedPA 505, dispersion compensating module 506 for compensating 80 km singlemode fiber dispersion and non-flattened later BA module 507 etc. In thisembodiment, at first calculate and define the followings: loss spectrumof 130 km transmission fiber 503, loss spectrum of non-flattened PA 505,insertion loss of dispersion compensating module 506 for compensating 80km single mode fiber dispersion, loss spectrum of non-flattened BA 507etc.

By calculating and measuring, the following curves can be obtained. FIG.6 shows loss spectrum of 130 km G.652 transmission fiber 503 at1530˜1560 nm wavelength band, and the non-flatness is about 0.8 dB. FIG.7 shows insertion loss difference between channels inserted bydispersion compensating module 506 for compensating 80 km fiberdispersion, and the non-flatness is about 0.32 dB. FIG. 4 shows outputpower differences for non-flattened PA 505, non-flattened BA 507,flattened BA 502. In FIG. 4, curve 401 represents the output opticalpower flatness of PA 505, curve 402 represents the output optical powerflatness of non-flattened BA 507, and curve 403 represents the outputoptical power flatness of BA 502 having been flattened. Then, throughcalculation, the insertion loss of transmission fiber 503, the outputoptical power of PA 505, the output optical power of BA 507 and theinsertion loss of DCM 506 used to compensate dispersion of the 80 kmfiber, in the receiving end unit 509, are all considered. Subtract lossspectrum characteristic curve of transmission fiber 503 and DCM 506 fromgain spectrum characteristic curve of PA 505 and BA 507, i.e. outputoptical power of whole unit before flattening=output optical power of PA505 before flattening+output optical power of BA 507 beforeflattening−insertion loss of 130 km G.652 transmission fiber503−insertion loss of DCM 506 for compensating dispersion of 80 kmfiber. The calculated power points set, at each wavelength, is a gainspectrum characteristic curve of the whole unit output optical powerbefore flattening.

At this moment, a complement curve of the combined gain spectrumcharacteristic curve is taken as the loss spectrum curve of GFF in thesystem, as shown in FIG. 8. In FIG. 8, the curve 801 is a GFF lossspectrum curve without considering the loss spectrum of transmissionfiber and dispersion compensating fiber, the curve 802 is a GFF lossspectrum curve with considering fiber affection. Comparing these twocurves can be seen that larger difference is at short wavelengths. TheGFF loss spectrum curve having been defined is used as a losscharacteristic target curve. A GFF coincides with the losscharacteristic target curve is manufactured and set in BA 507, i.e. theEFA of this embodiment. After deploying the said GFF, optical poweroutput curves of every channel are measured and shown in FIG. 9. FIG. 9shows comparison of the power equalization characteristics of everychannel before and after deploying optimized GFF, as shown by curves 902and 901 respectively. Obviously, considering loss spectrum of fiberlinks and optimizing loss characteristic curve of GFF better improvesthe channel optical power equalization property.

In the embodiment mentioned above, link repeatered units have not beenconsidered. FIG. 10 shows block diagram of a DWDM system with a linkrepeatered unit. For simplicity, the system is formed by adding a linkamplifier unit 1001 and a corresponding transmission fiber 1002 to theprevious embodiment. The link amplifier unit 1001 is consisted of PA1003, DCM 1004 and BA 1005. The transmission fiber 1002 and the linkamplifier unit 1001 compose the repeatered unit 1006. Other parts of thesystem can be divided into transmitting end unit 1007 and receiving endunit 1008, as in the previous embodiment, and the calculation method forthese two units is same as before.

In repeatered unit 1006, first calculate and define the followings: theloss spectrum of transmission fiber 1002, the gain spectrum ofnon-flattened PA 1003, the insertion loss of DCM 1004 for compensatingdispersion of single mode fiber, the gain spectrum of non-flattened BA1005. Then, subtract loss spectrum characteristics curve of transmissionfiber 1002 and DCM 1004 from gain spectrum characteristics curve of PA1003 and BA 1005, i.e. output optical power of whole unit beforeflattening=output optical power of PA 1003 before flattening+outputoptical power of BA 1005 before flattening−insertion loss oftransmission fiber 1002−insertion loss of DCM 1004. The calculated powerpoints set, at each wavelength, is a gain spectrum characteristic curveof whole unit output optical power before flattening. A complement curveof the combined gain spectrum characteristic curve is used as the targetcurve of GFF loss spectrum of the repeatered unit 1006. Manufacture aGFF which coincides the loss spectrum target curve and set in BA 1005,then gain balance of the repeatered unit 1006 is implemented. Combiningwith gain balance adjustment of the transmitting end unit 1007 and thereceiving end unit 1008 in the previous embodiment, gain balance of thewhole DWDM system can be implemented. A system with multiple repeateredunits can be dealt with the analogy of this.

All mentioned above are only the better embodiments of the invention,they are by no means to limit the protection scope of the invention.

What is claimed is:
 1. A method for implementing power equalization of aDWDM system comprising the steps of: a) measuring and calculating,respectively, a gain spectrum characteristic curve of an optical powerbooster amplifier unit and a loss spectrum characteristic curve of aloss device having related wavelengths with the optical power boosteramplifier unit in the DWDM system; b) subtracting the loss spectrumcharacteristic curve from the gain spectrum characteristic curve, thentaking the complement curve of the obtained difference curve as a losscharacteristic target curve of a GFF; c) setting in the optical powerbooster amplifier unit a GFF having loss characteristic curve coincidingwith the loss characteristic target curve.
 2. The method according toclaim 1 further comprising the steps of dividing, in advance, the wholeDWDM system into more than one independent unit at least having theoptical power booster amplifier unit; and calculating, respectively, theGFF loss characteristic target curves of each independent unit itself.3. The method according to claim 1 wherein the optical power boosteramplifier unit is an EDFA.
 4. The method according to claim 1 whereinthe loss device with relating wavelengths is a transmission fiber, or adispersion compensating module or their combination.
 5. The methodaccording to claim 4 wherein the dispersion compensating module includesdispersion compensating fibers.